Two new human herpesviruses have recently been described. The overall objective of this project is to better define their medical significance in immunocompromised patients in whom other herpesviruses are important pathogens. Specific Aim 1 To collect serial samples from the patients and test for the presence of HHV6 and HHV7 using qualitative PCR, quantitative PCR and methods to type strains of virus. Specific Aim 2 To use these virologic criteria to define infected patients so that the specificity and sensitivity of serologic methods for IgG antibodies can be determined. Specific Aim 3 To analyse all of the results obtained to identify temporal associations between the presence of either virus and particular clinical syndromes. To determine if these syndromes are associated with greater amounts of virus detected by quantitative methods. To determine the prognostic value for future disease of detecting either virus qualitatively or quantitatively in a prospective manner. Specific Aim 4 To determine whether infection or disease attributable to either virus is acquired from endogeneous reactivation of virus or from primary infection or reinfection from the donor organ in patients undergoing transplant. To determine if pre-existing immunity in the recipient can moderate the pathologic potential of these viruses. Overall, this project will define the medical significance of these new human herpesviruses for immunocompromised patients and will define when virus replication is at its highest so that trials of antiviral chemotherapy can be planned.